REVENUES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 779,344	$ 891,668	$ 2,347,907	$ 2,541,185
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	257,612	292,506	774,814	820,333
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	115,989	95,546	349,254	253,894
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	124,654	162,199	366,791	476,812
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	203,455	217,439	612,808	640,956
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	77,634	123,978	244,240	349,190
United States
Disaggregation of Revenue [Line Items]
Total revenues	666,649	764,352	2,044,523	2,188,327
Canada
Disaggregation of Revenue [Line Items]
Total revenues	112,695	127,316	303,384	352,858
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	527,970	586,872	1,591,246	1,658,970
Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	251,640	306,145	759,408	884,606
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	498,183	550,122	1,490,641	1,550,114
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	257,612	292,506	774,814	820,333
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	115,989	95,546	349,254	253,894
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	124,582	162,070	366,573	475,887
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Environmental Services | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Environmental Services | Oil, Gas and Lodging Services and Other
Disaggregation of Revenue [Line Items]
Total revenues		30,200		91,900
Operating segments | Environmental Services | United States
Disaggregation of Revenue [Line Items]
Total revenues	409,787	450,638	1,253,252	1,270,556
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	88,396	99,484	237,389	279,558
Operating segments | Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	281,089	341,417	857,048	990,146
Operating segments | Safety-Kleen | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	203,455	217,439	612,808	640,956
Operating segments | Safety-Kleen | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	77,634	123,978	244,240	349,190
Operating segments | Safety-Kleen | United States
Disaggregation of Revenue [Line Items]
Total revenues	257,048	313,979	791,919	917,240
Operating segments | Safety-Kleen | Canada
Disaggregation of Revenue [Line Items]
Total revenues	24,041	27,438	65,129	72,906
Corporate
Disaggregation of Revenue [Line Items]
Total revenues	72	129	218	925
Corporate | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	72	129	218	925
Corporate | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Oil, Gas and Lodging Services and Other
Disaggregation of Revenue [Line Items]
Total revenues		100		900
Corporate | United States
Disaggregation of Revenue [Line Items]
Total revenues	(186)	(265)	(648)	531
Corporate | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 258	$ 394	$ 866	$ 394